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31、无线电波（即赫兹波）。-----弹簧提供
赫兹波除微波波段兼用厘米表示它的波长外，一般均用频率代替波长，其单位为赫（Hz）。频率范围约在30千赫（kHz）～30000兆赫（MHz）之间。其波长范围在10-3～104米之间。当赫兹发现电磁波以后，首先被用于无线电信之传递试验。最早的无线电讯，借控制火花放电时间，构成电码讯号。火花放电是一种波长很短的减幅波，它的振幅衰减极快，且干扰极大，故不能用它做长距离通信。后经改良用电弧放电以获得长波的等幅波，使通信距离稍增，但其副波干扰仍不能免，且能量较小不能作远距离通信。后来俄国人波波夫与意大利业余无线电家马可尼同时独立地发明天地线制，马可尼且于天线中加接调谐电路，试验越过大西洋电码通信获得成功，至此无线电通信开始进入实用阶段。由于弧光电波的影响，误认为波长越长，电力越大，通信距离越远；事实上在短波通信特性未发现以前，确实如此。欧洲各国甚至竟用波长10000米以上，耗费巨资建设电台，并用频率很低的交流发电机供应电磁波能直接发射。长波无线电之传递，以地波为主。其折射率在海面与平原之吸收率均较小。在传播途中的衰减大致与距离成正比，因受气候影响甚微，在有效距离内通信可靠，故迄今仍在使用中，不过波长已减至1000米左右。长波无线电特别适宜于极地通信及海上导航，因短波无线电在极地失去电离层反射作用，无法达成远距离通信。长波通信须高大之天线。自超短波及微波通信实现后，已可利用轻便之转继站构成通信网，故长波通信已在淘汰。中波波段为标准广播波段，其波长自200～545米。适用于国内及邻接地区间作广播之用。国土广大之国家均规定其最大电力及广播服务区。最大电力以天线发射之电场强度为标准。中波广播波段自540～1650千赫（kHz）。中波广播也可用于空中导航。短波通信之特点是：即使是不太强的无线电波也能跨越大洋，而完成极远距离通信。短波无线电讯，由于频率较高，其电磁波由天线发出后，因地球表面矿物质之吸收率甚高，故不论发射电力多大，不出百里以内，其沿地面进行的电磁波即被吸收以尽，其衰减率以对数率随距离而加快。但在数百里以外的地区，因向空发射之天波受高空电离层的反射而再度出现。这种高空反射波的反射体是天空中大气因受紫外线辐射所形成的离子化气体层。围绕地球的这一层离子化气体称之为“电离层”。无线电波也和光波一样有四种特性：它能被物体吸收、反射、散射及折射。当电磁波前进时，遇金属物，则有一部分被吸收，有一部分即反射，且金属物与电子线波在不垂直又不平行之方向者，电磁波就发生散射。当电磁波经过不同的介质时，将发生折射的现象。
32、火山-----------boynton提供
世界上60％以上的活火山都分佈在太平洋沿岸新形成不久的巨大山脈和島嶼；另外有集中在地中海、非洲東部和大西洋中部的地殼發生巨大破裂的地方。因為這些區域的岩漿可以很容易的持續流出，所以成為活火山。
噴出的岩漿經過冷卻凝結，如果將火山口堵塞，則需要一段時間，在這段期間，地底的運動仍然活躍的進行著，因為火山口塞住，所以這股力量便在地底累積，等到累積到相當的程度，就又產生另一次的火山爆發，此種類型的火山就稱為休火山。
如果在地殼堅硬的地方，所有的裂縫都被堵住了，地底的岩漿再也不能爆發出來，這類的火山就稱為死火山。
33、季风---------弹簧提供
现代人们对季风的认识有了进步，至少有三点是公认的，即：（1）季风是大范围地区的盛行风向随季节改变的现象，这里强调“大范围”是因为小范围风向受地形影响很大；（2）随着风向变换，控制气团的性质也产生转变，例如，冬季风来时感到空气寒冷干燥，夏季风来时空气温暖潮湿；（3）随着盛行风向的变换，将带来明显的天气气候变化。
季风形成的原因，主要是海陆间热力环流的季节变化。夏季大陆增热比海洋剧烈，气压随高度变化慢于海洋上空，所以到一定高度，就产生从大陆指向海洋的水平气压梯度，空气由大陆指向海洋，海洋上形成高压，大陆形成低压，空气从海洋海向大陆，形成了与高空方向相反气流，构成了夏季的季风环流。在我国为东南季风和西南季风。夏季风特别温暖而湿润。
冬季大陆迅速冷却，海洋上温度比陆地要高些，因此大陆为高压，海洋上为低压，低层气流由大陆流向海洋，高层气流由海洋流向大陆，形成冬季的季风环流。在我国为西北季风，变为东北季风。冬季风十分干冷。
34、景深与光圈-----melody8818提供
http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/950/depth-of-field.html
http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/950/aperture-priority.html
35、Big eyed Bug Rearing----luoluo11提供
Natural History
Big-eyed bugs are true bugs in the order Hemiptera. The two most common species are Geocoris pallens and Geocorcis punctipes. Both are predators and occur in many habitats, including fields, gardens, and turf grass. Big-eyed bugs are considered an important predator in many agricultural systems and feed on mites, insect eggs, and small insects such as pink bollworm, cabbage loopers and whiteflies. Adult Big-eyed bugs are small (about 3 mm) black, gray, or tan with proportionately large eyes. Eggs are deposited singly or in clusters on leaves near potential prey. They develop with incomplete metamorphosis (there is no pupa) and take approximately 30 days to develop from egg to adult depending on temperature. Both nymphs and adults are predatory, but can survive on nectar and honeydew when prey are scarce. Big-eyed bugs, like other true bugs, have piercing-sucking mouthparts and feed by stabbing their prey and sucking or lapping the juices. Although their effectiveness as predators is not well understood, studies have shown that nymphs can eat as many as 1600 spider mites before reaching adulthood, while adults have been reported consuming as many as 80 mites per day.
Rearing
Big-eyed bugs are easy to rear. Adults can be purchased (see Suppliers) or collected in the wild by using a sweep net in a local alfalfa field, roadside or other lush area that may have potential insect prey. A Petri dish provides adequate housing when a piece of paper towel or filter paper is placed in the bottom and cut to fit the dish. Adults and nymphs should be housed separately as the larger individuals will eat the smaller ones. If you plan on raising large numbers of them, you may want to invest the time in making an artificial diet (see Cohen in the References), however they will also do well with fresh insect eggs (purchased), whitefly nymphs or aphids provided every few days. For moisture, place a one-inch piece of fresh green bean (replaced every couple of days) in the dish. A small piece of cotton placed in the adult cage provides a place for oviposition as well as a hiding place.  eriodically remove the cotton from the cage (eggs are pale orange) and place it in a new cage with a fresh piece of green bean. In a week to ten days, nymphs will emerge and a protein source (eggs, diet, etc...) will need to be added. When cages become dirty or moldy, transfer adults and nymphs to a fresh cage. Adults can be handled with soft forceps or with a small soft paintbrush. Nymphs should be handled with a soft brush only.
36、Giant Shield Volcanoes
The giant shield volcanoes on Mars are truly huge. The largest are three times as high as the biggest Earth volcanoes. They also are bigger in diameter. Thus, the biggest volcano on Mars is comparable to a pile of nearly 100 Hawaiian volcanoes. Despite this difference in size, the Mars shields look a lot like shield volcanoes on Earth. Both have the same broad flat profiles, large central calderas, and similar lava flow features. The giant martian shields are also much older than any Earth volcano. The youngest lavas on the martian shields are about 20 to 200 million years old. The oldest lavas are near 2.5 billion years old. Thus, these giant volcanoes were active for billions of years. This may explain their large size. On Earth, plate tectonics is always moving volcanoes away from their magma sources. Such movements are very slow, but they mean that most Earth volcanoes have distinct lifetimes. In the Hawaiian islands, for instance, volcanism lasts fo only a few million years on any given island. In contrast, the lack of plate tectonics on Mars allowed volcanoes to just keep growing. The only limit on their final size was the volume of lavas available.
37、A Brief History of the Guitar----luoluo11提供
There is evidence that a four string, guitar-like instrument was played by the Hittites (who occupied a region now known as Asia Minor and Syria) around 1400 BC. It had characteristically soft, curved sides--one of the primary features of anything identifiable as a guitar or predecessor. The Greeks also produced a similar instrument which was later modified by the Romans, though both versions appear to have lacked the curved sides. What is interesting here is that it seems this Roman cithara appeared in Hispania (now known as Spain) centuries before the Moorish invasion.
It had long been assumed that it was only after this invasion and the introduction of the Arabic ud in the South that a guitar-like instrument first appeared in Spain. But with the Roman cithara arriving centuries prior, we might say that although the ud influenced the development of the guitar it is not the true ancestor. According to this theory, the Spanish guitar derived from the tanbur of the Hittites, kithara with a "k" of the Greeks and then the cithara with a "c" of the Romans.
However, following the arrival of the Moors, the Roman cithara and the Arabic ud must have mixed and exerted mutual influences on one another for many centuries. Although there is no specific documentation, it is likely that makers of uds and citharas would have seen each other's work, if only through presentation by traveling troubadours. By 1200 AD, the four string guitar had evolved into two types: the guitarra morisca (Moorish guitar) which had a rounded back, wide fingerboard and several soundholes, and the guitarra latina (Latin guitar) which resembled the modern guitar with one soundhole and a narrower neck.
In the late 1400's, the vihuela was born by adding doubled strings and increasing its size. It was a large plucked instrument with a long neck (vibrating string length: 72 to 79 cm) with ten or eleven frets and six courses. It was the vihuela which became the preferred instrument of the Spanish and Portuguese courts and remained so until the late 1600's when orchestral and keyboard instruments became more prominent.
Although the guitar existed concurrently during this period, the vihuela and lute had overshadowed it until the end of the 17th century when the lute had acquired too many strings, was too hard to play and tune, and the vihuela was slowly replaced by the four and five course guitars (which had seven and nine strings respectively: one single high string, and three or four remaining courses--or pairs--of strings). It was perhaps the addition of the fifth course in the late 16th century that gave the guitar more flexibility and range and thus improved the potential of the repertoire that led to its ascent.
By the end of the 18th century and the beginning of the 19th, some guitars already used six single strings and employed fan struts under the soundboard. These struts were added for structural support to allow thinning of the top for greater resonance and for better distribution of sound across the board. Other contemporaneous developments included the use of a reinforced, raised neck using ebony or rosewood for the fingerboard, and the appearance of machine tuners in place of the wooden pegs. (It is noteworthy that the raised fingerboard had a great impact on the technique of the instrument since the strings were then too far from the soundboard to rest one's finger on the face for support.) These guitars would be unmistakably recognized by us as early classical guitars.
Beginning with the early 19th century, in the works of Agustin Caro, Manuel Gonzalez, Antonio de Lorca, Manuel Guiterrez from Spain and other European makers including Rene Lacote, and Johann Staufer, we find the direct predecessors of the modern classical guitar. By 1850, the guitar was prepared for its most important breakthrough since its inception, the work of Antonio Torres Jurado. With the encouragement of Julian Arcas and his own brilliant intuitions, Torres refined the strutting of the guitar to include as many as seven struts spread out like a fan under the soundboard. He increased the body size and the width of the neck considerably. These improvements allowed for greater volume and bass response as well as the development of a left hand technique for richer repertoire. The guitar was now prepared for the demands of the solo performer and the concert stage.
Although there have been continued developments since the middle 1800's, our modern guitar retains most of what was developed nearly 150 years ago. No one can say if we have reached the end of the evolution of the guitar, but until now, many of the best guitars from the point of view of volume, projection and sheer beauty of tone were made by the great makers, Torres, Ramirez and Arias from the second half of the last century!
38、insomnia----luoluo11提供
self-care techniques
in addition to the wise use of supplements and maintaining a healthy lifestyle, there are a number of other methods you can adopt to capture elusive sleep. you may find one particular technique does the trick or that a combination of several works best for you.
acupressure
many of my insomnia patients report significant relief when they do acupressure self-massage. stimulating specific points on your heel, wrist, and ankle can help to calm the nerves, relieve anxiety, and induce sound sleep. while you may feel immediately relaxed, it may take a month or more of doing acupressure daily to improve your troubled sleep patterns.
progressive muscle relaxation
both sleep researchers and the american psychological association have found progressive muscle relaxation to be beneficial for insomnia. in this exercise you learn what it feels like to relax by comparing relaxation with muscle tension. deep breathing is also an integral part of the technique.
bach flower remedies
flower remedies are a very safe and natural way to treat the anxieties, stress, and depression that often underlie insomnia. many brands of flower remedies are currently available, with virtually thousands of flower products from which to choose. i like the original extracts, known as the bach flower remedies (named for dr. edward bach, who originated the method in the early 20th century), which i find the most reliable.
there are 38 bach flower remedies, each one intended to treat a different negative emotion. for my patients struggling with insomnia, i often recommend white chestnut and scleranthus. white chestnut helps to chase away those obsessive worries that can keep you awake, while scleranthus dispels feelings of uncertainty and indecision. other bach flower remedies you might try are vervain, which fights tension; vine, to tackle the torments of ambition; and elm, to combat the sensation of being overwhelmed by responsibilities.
light therapy
our bodies have their own internal clocks called circadian rhythms, and when we tamper with them by flying across too many time zones or shifting working hours (or spending too much time indoors), they react by pulling the rug of sleep from under our toes.
while natural daylight is still the best therapy for light-deprived sleep disorders, such as jet lag, the sun is not always there when you need it. therefore i often suggest a light visor for my patients whose insomnia is caused by long-distance travel or shift work.
a light visor is fitted with a high-intensity full-spectrum bulb and worn on the head like a tennis visor. unlike its big brother the light box, the visor can be worn around the house while you do other things, and be stowed in a backpack or purse for traveling. many people find that while flying across times using a light visor to read helps to reset the body's biological clock even before the plane lands. (the directions that come with the light visor should clue you in on how to use the device during travel.)
studies also show that light therapy is very effective for those who can't fall asleep at night or who tend to wake up too early in the morning—a typical sleep pattern in older people.
39、Olympus Mons,--------redpassion和nos36提供
a gigantic (about 600 km/375 mi in diameter) shield volcano on Mars, is larger across than the length of the Hawaiian Islands strung together. The Mars Pathfinder Mission of 1997 returned data that Martian volcanic rocks appear to be similar to those found on Earth, including some evidence of the rock andesite. The volcanism on
加一下^^
the Earth's moon, Mars, Mercury, and Venus mostly occurred billions of years ago; these planetary bodies are now cold and dead. However, scientists have found evidence in Martian meteorites that indicates volcanic activity on Mars may have occurred as recent as 150 million years ago.
Mars has the largest volcano in the solar system, Olympus Mons. It is 26 km (16 mi) high (almost twice as high as Earth¡¦s Mount Everest) and covers an area comparable to the state of Arizona. Near it, three other volcanoes almost as large¡XArsia Mons, Pavonis Mons, and Ascraeus Mons¡Xform a line running from southwest to northeast. These four volcanoes are the most noticeable features of a large bulge in the surface of Mars, called Tharsis. Another volcano, Alba Patera, is also part of the Tharsis bulge, but is quite different in appearance. It is probably less than 6 km (4 mi) high, but has a diameter of 1,600 km (1,000 mi). None of Mars¡¦s volcanoes appear to be active.
The Tharsis bulge has had a profound effect on the appearance of the surface of Mars. The Tharsis bulge includes many smaller volcanoes and stress fractures, in addition to the large volcanoes. Its presence affects the weather on Mars and may have changed the climate by changing the rotation of the planet.
40、atom structure-------melody8818提供
http://www.cartage.org.lb/en/kids/science/Chemistry/Atoms/Atomic%20Stucture.htm
41、Sunflower,----------nos36提供
common name for annual and perennial herbs of a genus of the family of composite flowers. The genus, which contains about 67 species, is thought to be native to South America originally, although sunflowers are now distributed almost worldwide. Some of the tall-growing forms may attain a height of 3 m (12 ft). The large, solitary blossom, sometimes as large as a meter in diameter, is composed of yellow ray flowers and a central disk of either yellow, brown, or purple flowers, depending on the species. The daily orientation of the flower to the sun is a direct result of differential growth of the stem. A plant-growth regulator, or auxin, accumulates on the shaded side of a plant when conditions of unequal light prevail. Because of this accumulation, the darker side grows faster than the sunlit side. Thus, the stem bends toward the sun.
42、lost waxing方法制作sculpture----------nos36提供
Cire Perdue (French, ¡§lost wax¡§), process of wax casting used in making metal sculpture. A model is coated with wax. The solidified wax is encased in a two-layer mold of plaster or clay. It is then melted or otherwise removed from the mold, and metal is poured into the space where the wax had been. After cooling, the mold is broken to free the metal object. This ancient method is used to produce sculpture, jewelry, and utilitarian products such as dentures.
43、Surface Tension,----------nos36提供
condition existing at the free surface of a liquid, resembling the properties of an elastic skin under tension. The tension is the result of intermolecular forces exerting an unbalanced inward pull on the individual surface molecules; this is reflected in the considerable curvature at those edges where the liquid is in contact with the wall of a vessel. More specifically, the tension is the force per unit length of any straight line on the liquid surface that the surface layers on the opposite sides of the line exert upon each other. See Cohesion.
The tendency of any liquid surface is to become as small as possible as a result of this tension, as in the case of mercury, which forms an almost round ball when a small quantity is placed on a horizontal surface. The near-perfect spherical shape of a soap bubble, which is the result of the distribution of tension on the thin film of soap, is another example of this force; surface tension alone can support a needle placed horizontally on a water surface. See also Capillary Action.
44、微生物,----------davidfish提供
http://forum.chasedream.com/dispbbs.asp?boardID=19&ID=10583
45、Plant adaptation to the desert,---------- guangming 提供http://forum.chasedream.com/dispbbs.asp?boardID=19&ID=11702
46、the code of Hammurabi,----------popcorn提供
http://forum.chasedream.com/dispbbs.asp?boardid=19&id=11754&star=1#80150
47、古希腊三种柱子的详解,----------kanstory提供
http://forum.chasedream.com/dispbbs.asp?boardid=19&id=11765&star=1#80149
48、西方艺术流派,----------redpassion提供
http://forum.chasedream.com/dispbbs.asp?boardid=19&id=11769&star=1#80159
49、Maya Culture--------popcorn提供
The Maya are probably the best-known of the classical civilizations of Mesoamerica. Originating in the Yucatán around 2600 B.C., they rose to prominence around A.D. 250 in present-day southern Mexico, Guatemala, northern Belize and western Honduras. Building on the inherited inventions and ideas of earlier civilizations such as the Olmec, the Maya developed astronomy, calendrical systems and hieroglyphic writing. The Maya were noted as well for elaborate and highly decorated ceremonial architecture, including temple-pyramids, palaces and observatories, all built without metal tools. They were also skilled farmers, clearing large sections of tropical rain forest and, where groundwater was scarce, building sizeable underground reservoirs for the storage of rainwater. The Maya were equally skilled as weavers and potters, and cleared routes through jungles and swamps to foster extensive trade networks with distant peoples.
Around 300 B.C., the Maya adopted a hierarchical system of government with rule by nobles and kings. This civilization developed into highly structured kingdoms during the Classic period, A.D. 200-900. Their society consisted of many independent states, each with a rural farming community and large urban sites built around ceremonial centres. It started to decline around A.D. 900 when - for reasons which are still largely a mystery - the southern Maya abandoned their cities. When the northern Maya were integrated into the Toltec society by A.D. 1200, the Maya dynasty finally came to a close, although some peripheral centres continued to thrive until the Spanish Conquest in the early sixteenth century.
Maya history can be characterized as cycles of rise and fall: city-states rose in prominence and fell into decline, only to be replaced by others. It could also be described as one of continuity and change, guided by a religion that remains the foundation of their culture. For those who follow the ancient Maya traditions, the belief in the influence of the cosmos on human lives and the necessity of paying homage to the gods through rituals continues to find expression in a modern hybrid Christian-Maya faith.
Cosmology and Religion
The ancient Maya believed in recurring cycles of creation and destruction and thought in terms of eras lasting about 5,200 modern years. The current cycle is believed by the Maya to have begun in either 3114 B.C. or 3113 B.C. of our calendar, and is expected to end in either A.D. 2011 or 2012.
Maya cosmology is not easy to reconstruct from our current knowledge of their civilization. It seems apparent, however, that the Maya believed Earth to be flat and four-cornered. Each corner was located at a cardinal point and had a colour value: red for east, white for north, black for west, and yellow for south. At the centre was the colour green.
Some Maya also believed that the sky was multi-layered and that it was supported at the corners by four gods of immense physical strength called "Bacabs". Other Maya believed that the sky was supported by four trees of different colours and species, with the green ceiba, or silk-cotton tree, at the centre.
Earth in its flat form was thought by the Maya to be the back of a giant crocodile, resting in a pool of water lilies. The crocodile's counterpart in the sky was a double-headed serpent - a concept probably based on the fact that the Maya word for "sky" is similar to the word for "snake". In hieroglyphics, the body of the sky-serpent is marked not only with its own sign of crossed bands, but also those of the Sun, the Moon, Venus and other celestial bodies.
Heaven was believed to have 13 layers, and each layer had its own god. Uppermost was the muan bird, a kind of screech-owl. The Underworld had nine layers, with nine corresponding Lords of the Night. The Underworld was a cold, unhappy place and was believed to be the destination of most Maya after death. Heavenly bodies such as the Sun, the Moon, and Venus, were also thought to pass through the Underworld after they disappeared below the horizon every evening.
Very little is known about the Maya pantheon. The Maya had a bewildering number of gods, with at least 166 named deities. This is partly because each of the gods had many aspects. Some had more than one sex; others could be both young and old; and every god representing a heavenly body had a different Underworld face, which appeared when the god "died" in the evening
The Maya Calendar:
The Maya kept time with a combination of several cycles that meshed together to mark the movement of the sun, moon and Venus. The Maya calendar in its final form probably dates from about the 1st century B.C., and may originate with the Olmec civilization. It is extremely accurate, and the calculations of Maya priests were so precise that their calendar correction is 10,000th of a day more exact than the standard calendar the world uses today.
Of all the ancient calendar systems, the Maya and other Mesoamerican systems are the most complex and intricate. They used 20-day months, and had two calendar years: the 260-day Sacred Round, or tzolkin, and the 365-day Vague Year, or haab. These two calendars coincided every 52 years. The 52-year period of time was called a "bundle" and meant the same to the Maya as our century does to us.
The Sacred Round of 260 days is composed of two smaller cycles: the numbers 1 through 13, coupled with 20 different day names. Each of the day names is represented by a god who carries time across the sky, thus marking the passage of night and day. The day names are Imix, Ik, Akbal, Kan, Chicchan, Cimi, Manik, Lamat, Muluc, Oc, Chuen, Eb, Ben, Ix, Men, Cib, Caban, Eiznab, Cauac, and Ahau. Some of these are animal gods, such as Chuen (the dog), and Ahau (the eagle), and archaeologists have pointed out that the Maya sequence of animals can be matched in similar sequence to the lunar zodiacs of many East and Southeast Asian civilizations.
Writing and Hieroglyphics:
The Maya writing system is considered by archaeologists to be the most sophisticated system ever developed in Mesoamerica.
The Maya wrote using 800 individual signs or glyphs, paired in columns that read together from left to right and top to bottom. Maya glyphs represented words or syllables that could be combined to form any word or concept in the Mayan language, including numbers, time periods, royal names, titles, dynastic events, and the names of gods, scribes, sculptors, objects, buildings, places, and food. Hieroglyphic inscriptions were either carved in stone and wood on Maya monuments and architecture, or painted on paper, plaster walls and pottery.
The unit of the Maya writing system is the glyphic cartouche, which is equivalent to the words and sentences of a modern language. Maya cartouches included at least three or four glyphs and as many as fifty. Each cartouche contained various glyphs, as well as prefixes and suffixes. There is no Maya alphabet.
Maya writing is difficult to interpret for a number of reasons. First, glyphs do not represent just sounds or ideas, they can represent both, making it difficult to know how each glyph or cartouche should be read. In addition, many Maya glyphs can have more than one meaning, and many Maya concepts can be written in more than one way. Numbers, for example, can be written with Maya numerical symbols or with the picture of a god associated with that number, or a combination of the two. Some glyphs represent more than one phonetic sound, while also representing an idea. This means that a single idea can be written in many different ways. For example, the name of the Palenque ruler, Pacal, whose name literally means "Hand-shield", appears sometimes as a picture of a hand-shield, sometimes phonetically as pa-cal-la, and at other times as a combination of picture symbols and phonetics.
（http://www.civilization.ca/civil/maya/）
50、霓虹灯--------popcorn提供
A neon light is the sort of light you see used in advertising signs. These signs are made of long, narrow glass tubes, and these tubes are often bent into all sorts of shapes. The tube of a neon light can spell out a word, for example. These tubes emit light in different colors.
The idea behind a neon light is simple. Inside the glass tube there is a gas like neon, argon or krypton at low pressure. At both ends of the tube there are metal electrodes. When you apply a high voltage to the electrodes, the neon gas ionizes, and electrons flow through the gas. These electrons excite the neon atoms and cause them to emit light that we can see. Neon emits red light when energized in this way. Other gases emit other colors.
51、羊皮书制手抄本 --------popcorn提供
公元100年前后，古希腊人将纸莎草纸裁成单页，双面书写，写完后粘成类似今书本型。这 种文献，史称“手抄本”。手抄本翻阅方便，载文量大，具备了现代书的外型，逐渐成为图书的标准形式。
公元前800年左右，中东地区帕加马人，迫于亚历山大城对纸莎草的封锁以及希腊地区两大图书馆的竞争，在公元前2世纪发明了用羊皮、牛皮制成羊皮纸的工艺。 它将绵羊、山羊、羚羊、小牛或其它动物的皮进行加工处理，弄薄后，在其上书写文字。所 形成的古文献，史称“羊皮书”。羊皮书最初是书卷型的，后来演变为书本型。公元前200 年前后，帕加马成为羊皮纸的生产中心，并使该技术向各地传播。尔后羊皮纸的使用风靡罗马。
羊皮纸没有纸莎草那么笨重，而且可以折叠，成为“羊皮书”。公元1世纪，罗马人征服了地中海沿岸，没有建立起更大的图书馆，却建立了最大的档案管“Tabularium”。罗马的诗人可以在羊皮上 “发表”自己的作品，就是请专门的抄写手在羊皮上抄写多份并出售，尽管它在经济上并不合算。 中世纪的欧洲，基督教世界里的修道院开始也使用纸莎草，后来转到使用高级的羊皮纸，而且往往有精美的插图，抄写《圣经》成为修道士的一种职业；在阿拉伯世界里，他们则用羊皮纸抄写《古兰经》。公元4世纪前后，由于羊皮纸 坚固、书写清晰，而逐渐取代纸莎草纸成为制作手抄本的材料。
52、太阳系的行星-----redpassion提供
九大行星通常按以下几个方法分类：
根据组成：
固态的由石头构成的行星：水星，金星，地球和火星：
固态行星主要由岩石与金属构成，高密度，自转速度慢，固态表面，没有光环，卫星较少。
较大的气态行星：木星，土星，天王星和海王星：
气态行星主要由氢和氦构成，密度低，自转速度快，大气层厚，有光环和很多卫星。
冥王星。
根据大小:
小行星：水星，金星，地球，火星和冥王星。
小行星的直径小于13000公里。
巨行星：木星，土星，天王星和海王星。
巨行星的直径大于48000公里。
水星和冥王星有时被称作次行星(lesser planets)（不要与次级行星(minor planets)－－小行星的官方命名－－相混乱）。
巨行星有时被称为气态行星。
根据相对太阳的位置：
内层行星：水星，金星，地球和火星。
外层行星：木星，土星，天王星，海王星和冥王星。
在火星和木星之间的小行星带组成了区别内层行星和外层行星的标志。
根据相对地球的位置：
地内行星：水星和金星。
离太阳与地球较近。
地内行星看起来的如同地球上看有时不完整的月亮。
地球。
地外行星：火星到冥王星。
离太阳与地球较远。
地外行星看起来通常是完整的，或近乎完整的。
根据历史：
古典行星：水星，金星，火星，木星和土星。
史前即以得知
可用肉眼观测
现代行星：天王星，海王星，冥王星。
近现代所发现
用望远镜观测
地球
未知点：
太阳系是怎样起源的？一般来说是由尘粒与气体的星云压缩形成的，但详情很不清楚。
行星系统如何与其他星系共处？已有了木星般大小的在附近轨道运动的物件的恒星的极好的证据。组成固态行星的条件是什么？看起来地球这样的星体并不是独一无二的，但目前还没有直接证据证明这个或其他。
53、glaciers------江河湖海MM 提供
Glaciers exist where, over a period of years, snow remains after summer's end. They exist in environments of high and low precipitation and in many temperature regimes; they are found on all the continents except Australia and they span the globe from high altitudes in equatorial regions to the polar ice caps. There is a delicate balance between climatic factors that allows snow to remain beyond its season. ...
Scientists and skiers alike can note that within a few days of falling, snowflakes have noticeably begun to change. ... The snowflakes are compressed under the weight of the overlying snowpack. Individual crystal near the melting point have slick liquid edges allowing them to glide along other crystal planes and to readjust the space between them. Where the crystals touch they bond together, squeezing the air between them to the surface or into bubbles. During summer we might see the crystal metamorphosis occur more rapidly because of water percolation between the crystals. By summer's end the result is firn -- a compacted snow with the appearance of wet sugar, but with a hardness that makes it resistant to all but the most dedicated snow shovelers! Several years are usually required for the snow to settle and to season into the substance we call glacier ice. ...
We can best determine the health of a glacier by looking at its mass balance. Each year glaciers yield either a net profit of new snow, a net loss of snow and ice, or their mass may remain in equilibrium. Scientists divide each glacier into upper and lower sections termed the accumulation area, where snowfall exceeds melting during a year; and the ablation area, where melting exceeds snowfall. An equilibrium line, where mass accumulation equals mass loss, separates these areas. You can see it as the boundary between the winter's snow and the older snow or ice surface. Its altitude changes annually with the glacier's mass balance. To find mass balance, scientists measure the area of each region and observe amounts of accumulation and ablation relative to preset stakes. After density measurements are made they may calculate how much water has been added or lost to the glacier. ...
After a series of positive mass balance years, the glacier may respond to the increased thickness by making a glacial advance downvalley. A series of negative years may cause a glacial retreat, meaning that the terminus is melting faster than the ice is moving downvalley. ...
Glaciers have been likened to mighty rivers of ice. Although they move many times more slowly, glaciers have equivalent changes in flow rate and often form falls of fast-moving ice above slow-moving ice pools. Glaciers flow faster down their centers than at ice margins, and more quickly at the surface than at the bed. ...
How fast a glacier moves is mostly dependent on the thickness of the ice, and on the angle of its surface slope. Glacier speeds vary when changes are made in this geometry. They respond to excessively high seasonal snow accumulations by generating bulges of thicker ice that may move downvalley many times faster than the glacier's normal velocity. ...
54、黑洞-----wyxgeorge提供
“黑洞”很容易让人望文生义地想象成一个“大黑窟窿”，其实不然。所谓“黑洞”，就是这样一种天体：它的引力场是如此之强，就连光也不能逃脱出来。
根据广义相对论，引力场将使时空弯曲。当恒星的体积很大时，它的引力场对时空几乎没什么影响，从恒星表面上某一点发的光可以朝任何方向沿直线射出。而恒星的半径越小，它对周围的时空弯曲作用就越大，朝某些角度发出的光就将沿弯曲空间返回恒星表面。
等恒星的半径小到一特定值（天文学上叫“史瓦西半径”）时，就连垂直表面发射的光都被捕获了。到这时，恒星就变成了黑洞。说它“黑”，是指它就像宇宙中的无底洞，任何物质一旦掉进去，“似乎”就再不能逃出。实际上黑洞真正是“隐形”的，等一会儿我们会讲到。
那么，黑洞是怎样形成的呢？其实，跟白矮星和中子星一样，黑洞很可能也是由恒星演化而来的。
我们曾经比较详细地介绍了白矮星和中子星形成的过程。当一颗恒星衰老时，它的热核反应已经耗尽了中心的燃料（氢），由中心产生的能量已经不多了。这样，它再也没有足够的力量来承担起外壳巨大的重量。所以在外壳的重压之下，核心开始坍缩，直到最后形成体积小、密度大的星体，重新有能力与压力平衡。
质量小一些的恒星主要演化成白矮星，质量比较大的恒星则有可能形成中子星。而根据科学家的计算，中子星的总质量不能大于三倍太阳的质量。如果超过了这个值，那么将再没有什么力能与自身重力相抗衡了，从而引发另一次大坍缩。
这次，根据科学家的猜想，物质将不可阻挡地向着中心点进军，直至成为一个体积趋于零、密度趋向无限大的“点”。而当它的半径一旦收缩到一定程度(史瓦西半径)，正象我们上面介绍的那样，巨大的引力就使得即使光也无法向外射出，从而切断了恒星与外界的一切联系——“黑洞”诞生了。
与别的天体相比，黑洞是显得太特殊了。例如，黑洞有“隐身术”，人们无法直接观察到它，连科学家都只能对它内部结构提出各种猜想。那么，黑洞是怎么把自己隐藏起来的呢？答案就是——弯曲的空间。我们都知道，光是沿直线传播的。这是一个最基本的常识。可是根据广义相对论，空间会在引力场作用下弯曲。这时候，光虽然仍然沿任意两点间的最短距离传播，但走的已经不是直线，而是曲线。形象地讲，好像光本来是要走直线的，只不过强大的引力把它拉得偏离了原来的方向。
在地球上，由于引力场作用很小，这种弯曲是微乎其微的。而在黑洞周围，空间的这种变形非常大。这样，即使是被黑洞挡着的恒星发出的光，虽然有一部分会落入黑洞中消失，可另一部分光线会通过弯曲的空间中绕过黑洞而到达地球。所以，我们可以毫不费力地观察到黑洞背面的星空，就像黑洞不存在一样，这就是黑洞的隐身术。
更有趣的是，有些恒星不仅是朝着地球发出的光能直接到达地球，它朝其它方向发射的光也可能被附近的黑洞的强引力折射而能到达地球。这样我们不仅能看见这颗恒星的“脸”，还同时看到它的侧面、甚至后背！
“黑洞”无疑是本世纪最具有挑战性、也最让人激动的天文学说之一。许多科学家正在为揭开它的神秘面纱而辛勤工作着，新的理论也不断地提出。不过，这些当代天体物理学的最新成果不是在这里三言两语能说清楚的。有兴趣的朋友可以去参考专门的论著。
55、Idealism-----wyxgeorge提供
http://forum.chasedream.com/dispbbs.asp?boardID=19&ID=12767
[此贴子已经被作者于2003-8-27 19:02:39编辑过] |
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发表于 2003-6-18 18:49:00
